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A group of women employees is suing Walmart for discrimination, charging the nation's biggest retailer with underpaying female workers and denying them equal opportunities for promotion. Two federal courts have ruled that their suit can proceed as a class action on behalf of between 500,000 and 1.5 million women, but on Monday the Supreme Court announced it would review that decision. It looks suspiciously like another case in which the court's conservative majority will twist a procedural rule to prevent victims of discrimination from getting a fair chance at justice.

The women workers have set out a classic case of large-scale sex discrimination. They allege that women at Walmart and Sam's Club are paid less than men are even if they have more seniority and better performance reviews. They say women get fewer promotions and wait longer before getting promoted. (Read about Walmart's gender-discrimination suit.)

The women also put on evidence that executives used sexist language. Sam's Club managers, they say, often referred to women workers as "girls" or "little Janie Q's." And they put on evidence that about 65% of Walmart's hourly workers were women, while only about 33% of management employees were.

When the Supreme Court hears arguments in the case  most likely next spring  it will not hear the actual claims of discrimination. Instead, it will consider whether the courts below were right to allow the women to go forward on behalf of such a large class of employees and former employees. It is a procedural question, but the answer the court gives will have a big impact on the women's substantive rights. (See how Americans are spending now.)

Class action is a legal tool that makes it much easier for little-guy and little-gal victims of discrimination to sue. A female cashier at a Walmart in Oklahoma who is underpaid and unfairly passed over for promotion may not be able to find or afford a lawyer to represent her. But when a large number of women like her get together and sue as a class, they can usually get lawyers to take their case. They are also more likely to have the resources to put together the sort of complex proof  including statistical evidence and expert witnesses  necessary to win such a complicated case.

To proceed as a class, the women need to show that they are all affected by the same discriminatory policies. In the lower courts, the women successfully argued that Walmart's management practices are similar from store to store and that the company exercises a great deal of centralized control over its 3,400 U.S. stores. Walmart insists that the women are too diverse a group  working in too many places, in too many different job categories  to be considered a single group. The U.S. Court of Appeals for the Ninth Circuit agreed with the women, ruling that they could bring their suit as a large class action.